The present invention relates to dynamoelectric machines and, more specifically, to brushless exciters for large generators.
Large generators are driven by a prime mover to produce a supply of electric energy. A generator rotor is energized by an exciter providing to it a supply of DC power effective to produce a magnetic field rotating with the rotor. An annular stator surrounding the rotor contains a plurality of windings in which electricity is induced by the rotating magnetic field.
Providing the supply of DC power to the rotor involves transferring the DC power from a stationary element to the rotating element. One method for transferring the DC power includes slip rings rotating with the rotor and stationary brushes contacting the slip rings.
Slip-ring techniques are subject to reliability problems. An improved technique for transferring power from the stationary to the rotating element is conventionally known as a brushless exciter in which a DC field is applied to a stationary exciter winding. One or more windings rotating with the rotor pass through the magnetic field produced by the stationary exciter winding thereby producing AC power. The exciter AC power is rectified in a rectifier located on the rotor to produce the required DC excitation.
The amount of DC exciter power required by the rotor varies with the generator load. That is, as the generator load increases, the magnitude of the rotor magnetic field must be increased to maintain the desired output. This is conventionally accomplished by varying the amount of DC power fed to the stationary exciter winding. The DC power may be controlled by a control signal or in response to a measurement of the generator output voltage, optionally combined with a measurement of the generator output current.
An exciter field control responds to a drop in the generator output voltage with an increase in DC voltage fed to the stationary exciter winding. In some installations, the magnitudes and phase relationships of the output voltage and current are employed to compensate the exciter field voltage for the reactive component of the generator output.
Ideally, the exciter current should respond instantly to changes in generator load whereby the generator output voltage remains substantially constant. In practice, the response time of the exciter control circuit, and other factors, prevent attainment of this ideal with a conventional brushless exciter.